Atoms

• An atom is a unit of matter; the smallest unit of a chemical element.
• Each atom consists of a nucleus, which has a positive charge, and a set of electrons that move around the nucleus.
• Atoms link together to form molecules.
• An atomic clock is the most accurate clock available. Time is measured by the vibration of electrons in cesium atoms. The standard second is now defined by measurements on an atomic clock.
• An atomic number is the number of protons or electrons normally found in an atom of a given chemical element. The higher the atomic number, the heavier the atom is. In a neutral atom, the number of protons and electrons is the same.
• Atomic weight is the mass of a given atom, measured on a scale in which the hydrogen atom has the weight of I. Since most of the mass of an atom is in the nucleus, and each proton and neutron has an atomic weight of I, the atomic weight is very nearly equal to the number of protons and neutrons in the nucleus.
• A Bohr atom is the simplest modern picture of the structure of the atom, according to which electronics move in orbits around the nucleus.
• The electron's orbits can exist only at certain well-defined distances from the nucleus. When an electron changes orbits, it does so in a sudden quantum leap . The energy difference between the initial and final orbit is emitted by the atom in bundles of electromagnetic radiation called photons.
• A quantum leap is the movement of an electron from one orbit in an atom to another, sending out or taking on a photon in the process.